Abstract Art: Which Way is Up?

If an artist hasn't signed an abstract painting, or marked it with a "this way up" arrow on the back, how do you know which way to hang it up on a wall? And would it matter if you hung the piece 'upside down', if that was the way you liked it? Once you own a painting, isn't it your choice to hang it whichever way you like?

This is the debate that arose when my 'significant other' Alistair saw this painting of mine (which at that stage was unsigned and unvarnished). I had it the way up as it is here, but he said: "I bet you see it as fire, but I think it should be the other way up, as rain", and turned it upside down (see next page).